As new alloys are developed, corrosion characterization studies are of key importance in understanding material degradation processes and the performance of the alloy under corrosive conditions. The mechanisms by which alloy composition, microstructure, and passive films combine to improve passivity-based corrosion resistance were identified and considered in the design and synthesis of a novel Ni-Fe-Cr-Mo-W-Ru high entropy alloy (HEA). HEAs are a class of materials which are composed of five or more elements in nearly equi-atomic concentrations, which may lead to unique properties in regards to corrosion resistance and susceptibility1. Necessary components to the design of this HEA were the consideration of both passive film stability and breakdown, and pit stabilization in developing the alloy’s corrosion resistance2. Establishing regions of stability of the protective passive surface film in sulfate solution as a function of pH was the main goal of this study on a Ni-Fe-Cr-Mo-W-Ru HEA. This paper focuses on investigating the composition of oxide films as a function of potential and pH. Tests were conducted in Cl--free sulfate solutions3,4,5. Materials and Procedures Electrochemical corrosion testing of the Ni-Fe-Cr-Mo-W-Ru HEA in sulfate solutions was conducted in varying potentials and pH levels in order to evaluate the effect of potential and pH on the corrosion behavior of the alloy. Air-formed oxides were reduced by cathodic reduction followed by step anodic passivation to potentials in the passive range. In some cases, buffer solutions were used to control pH. In other cases, pH was adjusted using H2SO4 or NaOH in a base solution of 0.1 M Na2SO4. Though most HEAs by design consist of elements in equal proportion, the slightly higher proportion of Ni in this particular alloy made it appropriate to consider the Ni-Fe-Cr-Mo-W-Ru HEA as Ni-based. As such, the corrosion characteristics of the HEA in this study were compared to commercial solid solution Ni-based super alloys such as C-22 (Ni-Cr-Fe-Mo-Co-C-Mn-Si-P-S-V-W) and Alloy 600 (Ni-Co-Cr-Fe-C-Mn-Si-S-Cu), as well as other Ni alloys containing varying proportions of the passivity-promoting elements Cr, Mo, and W. Potentiodynamic, potentiostatic, and galvanostatic techniques were employed to characterize the electrochemical passivation behavior of the HEA, including oxide growth, dissolution, and potential step repassivation current densities as a function of E-pH. In-situ measurements of changes in oxide thickness were measured by single frequency electrochemical impedance spectroscopy (SF-EIS). Ex-situ characterizations of the HEA surface at the atomic level were conducted by XPS and Raman spectroscopy and in the future will be explored by 3-D atom probe methods. Acknowledgements This work was supported as part of the Center for Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584. References 1. Y. Shi, B. Yang, and P.K. Liaw, Metals, 7(2), 43 (2017). 2. G.S. Frankel, T. Li, and J.R. Scully, J. Electrochem. Soc., 164(4) C180-C181 (2017). 3. R.F. Reising, Corrosion, 31(5) 1975. 4. I. Yang, Corros. Sci., 33(1) 25-37 (1992). 5. G.O. Ilevbare, Corrosion, 62 (4) 340-356 (2006).
High entropy alloys (HEAs) contain five or more alloying elements typically in equimolar concentrations. In contrast, traditional solid solution alloys typically consist of one majority solvent element and several solute elements in comparatively low atomic proportions. HEAs are of interest for development of high performance materials particularly corrosion resistant alloys, because they can be engineered to form a single solid solution phase which reduces compositional and microstructural heterogeneity. Several hypotheses have been proposed to explain why HEAs form solid solutions, including a high entropy of mixing, and a near-zero enthalpy of mixing1 , 2. Many desired properties have been reported to arise in HEAs such as high hardness, superior wear resistance, high-temperature strength, and structural stability1 , 2. Excellent corrosion and oxidation resistance has been proposed but testing is limited1-3. HEAs are expected to perform superior to conventional alloys due to their unique combination of elements as well as an increase in the degrees of freedom in design which can be used to regulate the structure and properties of these alloys through alloying process control, and which cannot be achieved in typical binary and ternary systems3. This enhanced control over process variables in HEA design enables a systematic scientific examination of the effect alloy composition on corrosion properties, particularly passivity, metastable pitting, and repassivation. Characterization of the protective film formed on the HEA surface will provide an understanding of how alloy composition, microstructure, and oxide properties combine to improve passivity-based corrosion resistance and prevent film breakdown4. The focus of this talk will be to investigate alloy passivation as a function of chloride concentrations in an effort to elucidate the mechanism of aqueous corrosion in nickel-based HEAs, with the overarching goal of predictive corrosion-resistant alloy (CRA) design. In the present study a single HEA of composition 33.2Ni-16.85Cr-16.3Fe-8.57Mo-9.56Ru-5.48W, wt%, was designed and synthesized. The HEA demonstrated a stable FCC structure. The HEA was tested and compared to commercially pure Ni as well as the crystalline corrosion-resistant Ni-based alloy, C22 (Ni-21Cr-3.9Fe-13.3Mo-0.72Co-0.0035 C-0.23Mn-2.9W-0.026Si-0.011P-0.013V-0.0039S, wt%). Cyclic potentiodynamic polarization (CPP) scans were performed in solutions of increasing chloride concentration to assess the stability of the passive film. These tests enabled the identification of the passive current density, pitting, and repassivation potentials. A single frequency sinusoidal potential perturbation of 20 mV at 1 Hz was superimposed on the CPP scan to provide electrochemical impedance spectroscopy (EIS) data as a function of applied potential within the passive region. The electronic properties of the oxide film were also assessed using Mott-Schottky analysis of the EIS data. Ex-situ characterization was conducted to determine the corrosion morphology after electrochemical testing. The molecular identity of the oxide was determined using Raman and X-ray photoelectron (XPS) spectroscopies. Preliminary results indicated good corrosion resistance and a broad potential range for the passive region. Results were compared to pure Ni and C22. The passive range was compared to E-pH stability diagrams that were constructed using the CALPHAD approach and provided by Questek®. Acknowledgements This work was supported as part of the Center of Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584 References 1.Y. Qiu, M. A. Gibson, H. L. Fraser and N. Birbilis, Mater Sci Tech-Lond, 31, 1235 (2015). 2.M. H. Tsai and J. W. Yeh, Materials Research Letters, 2, 107 (2014). 3.Z. Tang, L. Huang, W. He and P. K. Liaw, Entropy, 16, 895 (2014). 4.E. McCafferty, Introduction to Corrosion Science, p. 1, Springer, New York, NY (2010).
Galvanic corrosion of 6061-T6 aluminum-coupled metals was studied in marine, volcanic, and rainforest environments. In addition to outdoor exposure, the galvanic couples were subjected to the chloride-containing GM 9540 accelerated corrosion test. The galvanic couple types included 6061-T6 Al paired with Ti-6Al-4V, 316 stainless steel, silver, copper, 1018 steel, or Mg AZ31B using fiberglass-epoxy fasteners. In this experiment, galvanic corrosion currents were measured through portable data loggers connected to each metal in the aluminum-coupled specimens. The total corrosion on the 6061-T6 Al in the galvanic couple resulted from the galvanic corrosion between the 6061-T6 Al anode and the cathode plus additional simultaneous local corrosion on the 6061-T6 Al caused by local cathodic reactions occurring on the 6061-T6 Al. The value of the total corrosion rate (i.e., local corrosion plus galvanic corrosion) was determined by mass loss of the galvanically coupled aluminum coupons. The local corrosion rate was determined using the difference between the total corrosion rate and the galvanic corrosion rate, as determined from the galvanic current data and Faraday’s law. The corrosion rates of the coupled 6061-T6 Al coupons were up to approximately 20 times greater than the uncoupled 6061-T6 Al coupons. Interestingly, less than 30% of the mass loss of the coupled 6061-T6 Al was directly due to the galvanic current, and accordingly 70% or more was due to local corrosion on the 6061-T6 Al. This implied that corrosion-product contaminants from the cathodic coupons (i.e., Ti-6Al-4V, 316 stainless steel, silver, copper, and 1018 steel) or byproducts such as excess H+ on the anodic 6061-T6 Al and excess OH- on the adjacent cathode may have had a major influence on the local corrosion of the 6061-T6 Al. In some cases, the type of cathodic coupon (i.e., Ti-6Al-4V, 316 stainless steel, silver, copper, and 1018 steel) made a significant difference on the corrosion rates of the 6061-T6 Al. Potentiodynamic polarization experiments were also conducted to study the mechanisms of galvanic corrosion for the couples described above.
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